Etching and mechanical grinding film-layers stacked on a semiconductor substrate
In some embodiments, a method of forming an integrated circuit includes providing a semiconductor substrate having an electronic circuit formed on a front side, and having a first material layer located over a second side of the substrate and a second material layer located between the first material layer and the second side. At least a portion of the first material layer is removed using a first chemical etching process, thereby exposing the second material layer. At least a portion of the second material layer is removed using a second chemical etching process. A portion of the substrate is then mechanically removed from the second side.
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Under 35 U.S.C. § 120, this continuation application claims the benefit of and priority to U.S. patent application Ser. No. 15/864,802, filed Jan. 8, 2018, the entirety of which is hereby incorporated herein by reference.
BACKGROUNDTechnological advancements have enabled the semiconductor industry to successfully scale down the sizes of transistors on integrated circuits, resulting in substantial reductions in integrated circuit (“IC”) sizes. In addition, improved manufacturing processes have allowed circuit designers and process engineers to increase the number of transistors on ICs by increasing the sizes of the wafers.
SUMMARYAccording to an embodiment, a method includes wet-etching a first film layer of a plurality of film layers stacked on a semiconductor substrate, the wet-etching of the first film layer performed using a first chemical, and the first film layer is an outermost film layer stacked on the semiconductor substrate. The method also includes wet-etching a second film layer of the plurality of film layers using a second chemical. Further the method includes using a mechanical grinding wheel to grind the semiconductor substrate to reduce a thickness of the semiconductor substrate.
According to another embodiment, a method of thinning of a semiconductor wafer includes protecting a front side of a semiconductor substrate using tape. The method further includes chemically etching a plurality of film layers stacked on the semiconductor substrate to expose the semiconductor substrate, the chemical etching performed using a first chemical selected to etch a first film layer and a second chemical selected to etch a second film layer. The method further includes grinding the semiconductor substrate to reduce a thickness of the semiconductor substrate.
According to yet another embodiment, a method includes providing a semiconductor substrate having an integrated circuit formed on a first side of the semiconductor substrate and having a silicon nitride film layer and a polysilicon film layer stacked on a second side of the substrate. The method further includes removing the silicon nitride film layer using a first chemical bath comprising 30%-49% hydrogen fluoride, inclusive. The method further includes removing the polysilicon film layer using a second chemical bath comprising 60%-70% nitric acid, inclusive and 0.5%-1% hydrogen fluoride, inclusive, to expose the semiconductor substrate. The method also includes thinning the semiconductor substrate using a mechanical grinder.
For a detailed description of various examples, reference will now be made to the accompanying drawings in which:
The size of an IC chip is usually not a concern when the chip is placed in an automobile, desktop computer, or other relatively large device. However, the size of the chip is of greater importance when the chip is placed in a miniature device, such as a smartphone. A reduction in the overall size of the IC package may be achieved using high-density packaging. High-density packaging is typically done by stacking multiple integrated circuits. Package density may be further improved by reducing the thickness of the wafer—used to fabricate a few thousand ICs at once—by performing wafer backgrinding (also commonly referred to as “wafer thinning”). Wafer backgrinding involves thinning semiconductor wafers after ICs have been fabricated onto the wafer.
Mechanical wheel grinders are employed to remove unnecessary silicon substrate from the backside of the semiconductor wafer. In some cases, the backside of the semiconductor wafer includes an extraneous stack of film layers, which leads to an increase in the overall thickness of the high-density package. In such cases, a reduction in the thickness of the wafer is achieved by removing both the extraneous film layers and a portion of the silicon substrate from the backside of the wafer by mechanical wheel grinders. In some cases, the film layers on the backside of the semiconductor wafer are grind-resistant and/or hard such that the mechanical wheel grinders are damaged during the grinding process. Replacement of a damaged mechanical wheel grinder is expensive and performing semiconductor wafer thinning using a damaged wheel can further damage the semiconductor wafer. Thus, there is a need in the art for methods to prevent damage to the mechanical wheel grinders.
The present disclosure provides techniques to prevent damage to mechanical wheel grinders during the removal of film layers stacked on the backsides of semiconductor wafers. More specifically, the present disclosure relates to replacing the mechanical wheel grinding technique employed to remove the film layers present on the backsides of the wafers with a wet-etching technique. In some embodiments, backside wafer thinning is performed by first wet-etching the film layers and then wheel-grinding the semiconductor substrate to thin the substrate. In this way, the mechanical wheel grinder is not used to remove film layers—some of which have hardness characteristics equivalent to those of a diamond—from the backside of the semiconductor wafer, and so the integrity of the mechanical wheel grinder is preserved.
Fabrication processes are performed on a front side 110 of the semiconductor wafer 100 to produce a plurality of integrated circuits (not expressly depicted in
The inverted side view of the portion 120 of the wafer 100 includes a semiconductor substrate 200. The front side 110 refers to a surface of the substrate 200 opposite the film layers 205, 210, 215 and 220. The backside 115 refers to the surface of the outermost film layer 205, 210, 215, 220 opposite the substrate 200. More specifically, in
The film layers 205, 210, 215 and 220 may also be intentionally formed on the substrate 200 using fabrication processing techniques. In particular, semiconductor wafer fabrication processing can include many process steps, such as surface cleaning, epitaxy, oxidation, diffusion/implantation, photolithography, etching, layer deposition, multiprobe, and backgrinding. These steps often involve reactive chemicals such as hydrofluoric acid, which is particularly useful in etching oxides and nitrides. Furthermore, various manufacturers supply the equipment used in wafer production, and there is no standard on how the equipment interfaces with the wafer. As such, a large variety of handling systems are used that have little in common with each other except that they all handle the wafer. Thus, it is important that the wafer be designed to withstand the fabrication process in order to support high yield and profitability. The problem noted above is in large part addressed by coating the wafer with a thin film or coating. Thus, in some embodiments, the film layers 205, 210, 215, 220 are necessary and intentionally grown on the backside of the wafer as they provide mechanical strength to the wafer for the processes to be performed on the front side 110 to fabricate the IC. For example, the backside of the semiconductor wafer 100 can be covered with silicon nitride, silicon dioxide, polysilicon etc. In some embodiments, the film layers 205, 210, 215, 220 prevent dopants from diffusing out of the substrate 200. The methods of producing silicon nitride include nitriding and low pressure chemical vapor deposition (LPCVD). Other CVD techniques can be used to generate film layers on the backside of the semiconductor wafer.
Referring again to
In some embodiments, the number of film layers on the semiconductor substrate 200 can vary and may not be present in the same number as depicted in
The method 300 begins in step 310 with wet-etching a first film layer of the plurality of film layers 205, 210, 215, 220 stacked on the backside of the semiconductor substrate 200 using a first chemical. The first layer to be wet-etched will be the outermost layer, and in this embodiment the outermost layer is marked 220. Consider, as an example, that the film layer 220 is a silicon nitride layer. The silicon nitride layer can be wet-etched using a chemical bath including 30-49%, inclusive, of hydrogen fluoride. In other embodiments, the film layer 220 is an oxide layer which can also be wet-etched using a chemical bath including 30-49% hydrogen fluoride, inclusive. In another embodiment, the film layer 220 may be a polysilicon layer which can be wet-etched using a chemical bath including 60-70% nitric acid, inclusive and 0.5-1% hydrogen fluoride, inclusive. Wet-etching of the film layer 220 may continue until the film layer 220 is completely removed.
The method 300 continues in step 320 with wet-etching a second film layer. With respect to
As mentioned above, a reduction in the thickness of the wafer 100 is achieved by removing the film layers by wet-etching and by removing a portion of the silicon substrate 200 by mechanical wheel grinding. Step 310 and 320 remove extraneous film layers (e.g., film layers 205, 210, 215 and 220) from the semiconductor substrate 200. Once the silicon substrate is exposed, the method of claim 300 further continues in step 330 with backgrinding the semiconductor substrate 200 (
The method 400 begins in step 410 with protecting the front side 110 of the semiconductor wafer 100 using tape. Any suitable type of tape, such as UV (ultraviolet)-curable backgrinding tape, or a non-UV tape, may be used to ensure protection against wafer surface damage during backgrinding and to prevent wafer surface contamination caused by infiltration of grinding fluid and/or debris. The method 400 continues in step 420 with chemically etching a plurality of film layers stacked on the substrate 200. As noted above, the number of film layers present on the substrate 200 may depend on the mechanical strength needed by the semiconductor wafer 100 for the processes to be performed on the front side 110 to fabricate the ICs. As noted above, in some cases, the film layers may also be present to avoid doping contamination (“back-seal” films). Additionally, in some cases, the number of film layers present on the substrate 200 depends on the fabrication processes performed on the frontside 110.
The chemical etching of step 420 is selective in nature. In particular, the film layers are selectively etched using chemicals capable of etching each of the film layers 205, 210, 215, and 220. For example, a film layer can be a silicon nitride layer, a silicon oxide layer, a polysilicon layer, etc. Certain chemicals selectively etch—one layer at a time—each of the above-mentioned film layers. For instance, a chemical bath including 30-49% hydrogen fluoride, inclusive, can be used to remove a silicon nitride layer and a silicon oxide layer and a chemical bath including 60-70% nitric acid, inclusive, and 0.5-1% hydrogen fluoride, inclusive, can remove a polysilicon layer. The chemical bath removes the topmost layer and the removal process essentially stops when the topmost layer is removed as the chemical bath used is a selective etchant. Thus, for example, if the topmost layer is silicon nitride, the chemical bath including 30-49% hydrogen fluoride will be utilized, but this chemical bath will cease etching once the silicon nitride layer is removed. Assume, as an example, that after removing the silicon nitride layer, the polysilicon layer becomes the topmost layer. A different chemical bath including 60-70% nitric acid, inclusive, and 0.5-1% hydrogen fluoride, inclusive, can be used to etch the polysilicon layer. In this manner, the step 420 chemically removes the plurality of backside film layers until the semiconductor substrate 200 is exposed. In some embodiments, the chemical bath including 60-70% nitric acid, inclusive, and 0.5-1% hydrogen fluoride, inclusive, can be utilized to remove a portion of the substrate 200.
The method 400 further continues in block 430 with backgrinding the semiconductor substrate 200. As noted above, a reduction in the thickness of the semiconductor wafer 100 is achieved by removing the film layers 205, 210, 215, and 220 by wet-etching and by removing a portion of the substrate 200 by mechanical wheel grinding. Backgrinding is performed using a mechanical grinder and in this embodiment, the backgrinder removes a portion of the semiconductor substrate 200. In some embodiments, to improve the productivity of the grinding operation, a multi-step grinding operation is generally performed. The first step uses a large grit to coarsely grind the wafer and to remove a bulk of the excess substrate thickness. A finer grit is then used in the second step to polish the substrate and to accurately grind the substrate to the desired thickness.
The method 500 continues in step 530 with removing the exposed layer, i.e., the polysilicon layer, using a different chemical bath including 60-70% nitric acid, inclusive, and 0.5-1% hydrogen fluoride, inclusive. The method of claim 500 continues in step 540 with backgrinding the exposed semiconductor substrate 200. Backgrinding is performed using a mechanical grinder to remove a portion of the semiconductor substrate 200. In some embodiments, to improve the productivity of the grinding operation, a multi-step grinding operation is generally performed. The first step uses a large grit to coarsely grind the substrate and to remove a bulk of the excess substrate thickness. A finer grit is used in the second step to polish the substrate and to accurately grind the substrate to the desired thickness. In some embodiments, the chemical bath including 60-70% nitric acid, inclusive, and 0.5-1% hydrogen fluoride, can be utilized to remove a portion of the substrate 200.
The above discussion is meant to be illustrative of the principles and various embodiments of the present disclosure. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.
Claims
1. A method of forming an integrated circuit, comprising:
- providing a semiconductor substrate having an electronic circuit formed on a first side of the substrate, and having a first material layer located over a second side of the substrate and a second material layer located between the first material layer and the second side;
- removing at least a portion of the first material layer using a first chemical etching process, thereby exposing the second material layer;
- removing at least a portion of the second material layer using a second chemical etching process; and
- then mechanically removing a portion of the substrate from the second side.
2. The method of claim 1, wherein removing at least a portion of the second material layer at least partially exposes the second side of the substrate.
3. The method of claim 1, wherein the first material layer comprises silicon nitride and the first chemical etching process includes exposing the silicon nitride to a wet-etching bath.
4. The method of claim 3, wherein the wet-etching bath includes 30%-49% hydrogen fluoride, inclusive.
5. The method of claim 1, wherein the first chemical etching process removes the first material layer at a greater rate than the first chemical etching process removes the second material layer.
6. The method of claim 1, wherein the first material layer comprises polysilicon and the first chemical etching process includes exposing the polysilicon to a wet-etching bath.
7. The method of claim 6, wherein the wet-etching bath includes 60%-70% nitric acid, inclusive and 0.5%-1% hydrogen fluoride, inclusive.
8. The method of claim 6, wherein the first material layer includes a silicon oxide sublayer and the first chemical etching process further includes removing at least a portion of the silicon oxide sublayer.
9. The method of claim 1, wherein the first chemical etching process includes wet-etching the first material layer with a solution comprising 60%-70% nitric acid, inclusive and 0.5%-1% hydrogen fluoride, inclusive.
10. The method of claim 1, wherein the first chemical etching process includes wet-etching the first material layer with a solution comprising 30%-49% hydrogen fluoride, inclusive.
11. The method of claim 1, wherein the second first material layer includes polysilicon, and the first chemical etching process removes the first material layer and stops on the polysilicon.
12. The method of claim 1, wherein the first material layer includes silicon nitride, and the first chemical etching process removes the first material layer and stops on the silicon nitride.
13. The method of claim 1, wherein the first and second material layers are derived from a plurality of microfabrication processes performed on the first side of the semiconductor substrate.
14. The method of claim 1, further comprising protecting the first side with tape while mechanically removing the portion of the substrate from the second side.
15. The method of claim 1, wherein the mechanically removing includes backgrinding the second side using a mechanical grinding wheel.
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Type: Grant
Filed: Feb 22, 2019
Date of Patent: Feb 18, 2020
Patent Publication Number: 20190214263
Assignee: TEXAS INSTRUMENTS INCORPORATED (Dallas, TX)
Inventors: Jian Jun Kong (Sichuan), She Yu Tang (Sichuan), Tian Yi Zhang (Sichuan), Qin Xu Yu (Chengdu), Sheng Pin Yang (Chengdu)
Primary Examiner: Roberts P Culbert
Application Number: 16/282,344
International Classification: H01L 21/304 (20060101); H01L 21/311 (20060101); H01L 21/683 (20060101); H01L 21/3213 (20060101);